Strawberries

General Information

Strawberries are attacked by a variety of pests, including insects, mites, pathogens and weeds. While much of this publication deals with chemical controls, the best overall approach to pest management integrates chemicals with other methods. Where possible, cultural practices that may help in managing these pests are presented.

The single most important factor in controlling pathogens is the maintenance of vigorously growing plants. Weeds compete with strawberries for essential water and nutrients. Weeds also promote pest injury by acting as alternate ‘homes’ for diseases and insects, inhibiting spray penetration, and maintaining high humidity in the strawberry leaf canopy.

Good soil and air drainage are essential for plant health. Roots rot quickly in waterlogged soil, and fruit rots are more common when the soil surface does not dry quickly. Well-drained loams are the most suitable soil types for good root penetration and plant growth. Sites where cold air can drain away to lower levels will decrease the possibility of frost damage to the flowers and fruit. A southern, sloping site is the most ideal location providing quick-drying soil and earlier ripening berries.

For good root penetration, aeration and drainage, organic materials should be added to the soil. Disc animal manures, compost, and/or green manure crops (cover crops) thoroughly into the soil before planting. The use of leguminous cover crops may increase soil nematode populations, which may be injurious to strawberries. Sudan grass (which will suppress nematode populations) and Japanese millet are annual cover crops well suited for most situations, providing heavy organic matter production. See section on “Cover Crops and Green Manures” on page 8 for more on this subject. If poultry manure is used, it must be applied cautiously. It is a rich source of nitrogen and phosphorus which, if used to excess, can promote excessive vegetative growth and soft berries (both conditions encourage disease), and may leach into ground water.

In new beds, a soil test should be done to determine the pH, and the rate and types of fertilizer to apply. Have the soil tested at your state university or private soil-testing lab and apply the necessary lime to adjust the pH to within the range of 5.8 to 6.2. Some soils low in magnesium may benefit from the use of dolomitic (Hi-Mag) lime. Pre-plant fertilizer recommendations will generally call for the application of blended fertilizer containing nitrogen, phosphorous and potash in a 1-2-2 ratio (250 to 400 pounds of 10-20-20 is a typical recommendation). Nitrogen at up to 30 pounds per acre banded over the plant row is generally recommended during the period of heavy runner development (late June or early July). An additional, smaller application (up to 20 lbs N) may be suggested for early August.

Heavy fertilizer applications should be avoided in the spring on established beds; too much nitrogen will promote abundant vegetative growth that encourages disease by inhibiting good air circulation needed to dry plant surfaces. The longer moisture films remain on fruit and leaves from irrigation, rain, dew or high humidity, the greater the chance of fungal spores germinating and disease outbreaks occurring. Berries may also become soft as a result of too much nitrogen. Light applications of fertilizer may be made in spring (8-15 lbs of actual N per acre) to promote early plant growth and fruit development.

Leaf tissue analysis is a good way to determine nutrient levels actually in the plant rather than what is in the soil. Sometimes the nutrients in the soil are not available to the plant due to pH, organic matter content, or some other reason. Leaf tissue analysis tells you what the plant is getting and what the plant is lacking. The samples are taken after bed renovation in the summer from the first fully expanded new leaves. At least 50 complete leaves per planting should be taken, rinsed, and allowed to dry completely before processing. Contact your regional fruit specialists for the exact protocol, processing instructions, and fees. Standards are available for comparison to determine if your results indicate the need for corrective measures. See Table 13.

Good root development is essential to the continued productivity and health of the strawberry planting. Primary roots generally live only a year or slightly longer, requiring the development of new roots at successively higher nodes on the growing crowns. To encourage increased root development, strawberry crowns are mulched with about 1 inch of loose soil during the renovation process, enough soil to cover the crown extension that has occurred during the past year without covering the top of the crowns.

Strawberries are a cool weather crop, producing most of their growth in the spring and fall. Growth is greatly slowed during the hot, dry summer months, resulting in a shallow root system. During the growing season (April, May, August, September and October) applying 1-1/2” of water every 12 to 14 days will aid in maximum growth and fruit bud development. During fruiting, adequate moisture (1" to 2” of water per week) will maintain fruit size and production.

Irrigation can also eliminate frost damage to flowers during early bloom periods. If sprinklers are turned on before the temperature at ground level drops to 32˚F and continued until air temperature is above freezing and all ice has melted off the plants, the blossoms will be protected. (Remember, the first blossoms to open will bear the largest berries.) The sensitive, actively growing tissue in the crown will also be protected from freezing injury that would make it more susceptible to pathogen attack.

In Row Spacing

Spacing between Rows

Table 14. Number of strawberry plants per acre at different spacings.

36 inch

40 inch

42 inch

6 inches

29,040

26,241

24,891

12 inches

14,520

13,120

12,446

18 inches

9,680

8,712

8,297

24 inches

7,260

6,540

6,223

Stage of Development

Approx. Critical Temperature

Table 15. Critical freeze temperatures for strawberries based on stage of growth.

Tight bud

25˚F

"Popcorn"

26˚F

Open Blossom

30˚F

Fruit

28˚F

Deficient

Below Normal

Normal

Above Normal

Excessive

Table 16. Critical nutrient values for strawberry tissue analysis.

N (%)

1.50

1.80

2.00

2.80

>2.80

P (%)

0.20

0.25

0.35

0.40

>0.50

K (%)

1.20

1.50

2.00

2.50

>3.00

Ca (%)

0.60

0.70

1.50

1.70

>2.00

Mg (%)

0.25

0.30

0.45

0.50

>0.65

Mn (ppm)

40

50

150

200

>250

Fe (ppm)

50

60

150

250

>300

Cu (ppm)

5

7

10

20

>25

B (ppm)

20

30

60

70

>85

Zn (ppm)

15

20

35

50

>60

Day Neutral Strawberries

Day neutral strawberries set fruit continually over several weeks, during the late summer and fall, providing a high value specially item for roadside stands and farm markets. Day neutral plants are often grown as an annual crop, on raised beds with plastic mulch and very high planting densities. As a result, the establishment costs and labor commitment tend to be quite high.

Fertilizer should be applied and worked into the soil prior to planting, or banded into the soil prior to applying plastic mulch and planting. Rates should be determined through soil test but a standard rate of 100 lbs of nitrogen, 50 lbs of phosphorus (P205) and 50 lbs of potassium (K20) is typically incorporated into the soil prior to planting (e.g. 600 lbs/acre of 20-10-10 or its equivalent). Beds should be prepared in the spring or made during the previous fall. Raised beds should be 4” to 12” high with a 1 ½” crown sloping from the center to the edges of the bed to shed water. Bed width depends on how many rows of plants will be established on each bed, from 18” for a single plant row to 46” for 3 to 4 plant rows. In New England, two plant rows per bed are most common and simplest to manage with the bed with of 24” to 42”. Smooth, well packed and shaped beds greatly improve the fit and performance of plastic mulch. Trickle or drip irrigation lines should be installed during bed forming at about a 4” depth in the bed, and a few inches to the side of the planet rows, with either one or two lines, depending on how many plant rows there will be on the bed. Plastic mulch should be laid tightly over the bed immediately after bed forming. Black plastic is commonly used to promote soil warming and to provide weed control.

Dormant, day neutral strawberry crowns should be planted in the spring as soon as the beds are prepared. Planting is done by hand using a simple planting tool. A piece of 1/8” iron flat bar about 12” long is bent at a 90° angle about 4” from one end to create a handle this and is often wrapped with duct tape to provide a soft grip. The opposite end of the bar is notched from the edges to the middle to about a ¾” depth to create shallow inverted “V” at the end. The notched edge is slightly sharpened to ease penetration through the mulch and soil. The notched edge of the tool is placed over a plant that is laid on the plastic so that it will ”grab” about ½” of the root ends as the tool is pushed into the soil, drawing the plant into the bed. Push the crowns straight down through the mulch with the tool and into the soil so that the soil surface comes halfway up the crown. Gently pinch the soil around the crown as you withdraw the planting tool. Plants should be spaced 10” to 14” apart within a row. Planting in a double row, 24” apart, on a 42” wide bed with 13” between plants within the row will require about 13,400 plants per acre.

All flower blossoms that emerge should be pinched off for 4 to 6 weeks after planting. All runner plants that emerge during the summer should also be removed. While runner removal is labor-intensive, it improves both yield and fruit quality. The field should be irrigated immediately after planting and regularly thereafter. One to 2” of water per week is recommended. Drip lines can be used to deliver soluble fertilizers to the plant. Recommended rates of fertilizer vary depending on the number of plants per acre, soil type, and variety. Generally, 2 lbs of actual nitrogen per acre per week applied through the drip lines will provide good growth.

Day neutral strawberry beds are not usually carried over for second year. Although plants can produce an early spring crop the following year, and fruit again the next summer and fall if carried over, fruit quality, especially size, is much lower and runner control becomes a major problem. If the beds are to be carried over, winter protection is required in the form of heavyweight row covers, applied in the fall when the plants are dormant.